The present invention relates to an image forming method of the type transferring a toner image from an image carrier to a recording medium via an intermediate image transfer belt, or intermediate image transfer body, or transferring it from the image carrier to a recording medium carried on a transfer belt, or medium carrier, and a copier, printer facsimile apparatus or similar image forming apparatus for practicing the same.
An image forming apparatus of the type transferring a toner image from a photoconductive element or image carrier to an intermediate image transfer belt (primary transfer) is well known in the art. For the primary transfer, use may be made of an indirect bias application system, which applies a bias for image transfer indirectly to the belt. In the indirect bias application system, a bias roller for belt transfer is positioned downstream of a nip between the photoconductive element and the belt while a ground roller is positioned upstream of the nip. The above bias is applied to the bias roller in order to transfer a toner image from the photoconductive element to the belt.
The problem with the above image forming apparatus is that toner is scattered at the time of primary transfer of a toner image from the photoconductive element to the belt. Specifically, at the time of primary transfer, a toner image formed on the photoconductive element is not transferred to a preselected position on the belt, but is scattered around the preselected position and blurred. Particularly, such scattering of toner causes thin lines to lose sharpness.
One cause of the scattering of toner is so-called pretransfer, i.e., the transfer of toner from the photoconductive element to the belt occurring at a position upstream of the nip between the element and the belt in the direction of movement of the element, as well known in the art. Another cause is so-called retransfer, i.e., the transfer of toner from the belt back to the photoconductive element occurring at a position downstream of the above nip. More specifically, as for pretransfer, when the bias is applied to the bias roller, a potential slope occurs between the bias roller and the ground roller and forms an electric field even at the side upstream of the nip, causing the toner to move toward the belt away from the photoconductive element. As for retransfer, the toner image successfully transferred from the photoconductive element to the belt is disturbed by an electric field for image transfer formed at the side downstream of the nip.
Presumably, the above pretransfer and retransfer also occur when a toner image is directly transferred from the photoconductive element to an image transfer belt used to convey a recording medium.
It is a common practice with an image forming apparatus using the intermediate image transfer belt or the transfer belt to cause the belt to contact an object facing it by use of a pressing member. The pressing member presses the surface of the belt opposite to the surface expected to contact the object. However, with this kind of arrangement, it is likely that when the belt is left unused over a long time, both the belt and the object contacting each other over a long time are damaged, and the belt curls complementarily to the contour of the pressing member. The cur led portion of the belt would vary the mechanical contact condition and therefore the image transfer condition on entering the nip and would thereby bring about a defective image ascribable to, e.g., irregular image transfer.
The above problem arises not only in an image forming apparatus including the image transfer belt or the transfer belt, or image transfer body, to which a toner image is transferred from the image carrier, but also in an image forming apparatus including a belt, a pressing member for pressing the belt, and an object which the surface of the belt opposite to the surface pressed by the pressing member contacts.
Technologies relating to the present invention are disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 5-249842, 8-166731, 8-2409591, and 10-161440.